Without limiting the scope of the present invention, its background will be described with reference to controlling the operation of a down-hole safety valve. An important consideration involved in the operation of hydrocarbon fluid wells is the ability to cut off the flow of fluids or gas in the event of emergencies, equipment problems or similar situations.
An important consideration in the selection of surface controlled subsurface safety valves is that valve closure be essentially failsafe. Consequently, subsurface safety valves are normally configured to be in a closed position absent operator control. Typically a subsurface safety valve is biased to the closed position through the use of one or more springs, configured to close the valve in the event that operator control is lost. In the case of hydraulically controlled rod-piston type safety valves, the valve is opened with the application of hydraulic pressure to a piston which actuates the valve, positioning it in an open position. If control pressure is lost the valve closes.
Control of conventional hydraulically operated, spring loaded rod-piston type downhole safety valves is, however, limited by the hydrostatic force applied to the piston. Ideally, a hydraulically controlled downhole safety valve would be designed to operate over a wide range of downhole locations, independent of the depth at which the valve is positioned. However, the hydrostatic force applied by the column of fluid in the control line varies with the depth at which the valve is positioned while the counteracting spring force biasing the safety valve closed is constant. Thus, if the valve is positioned at a depth such that the hydrostatic pressure or force generated by the column of fluid in the control line or tube is greater than the biasing force exerted by the spring mechanism, the valve will not close in response to a decrease in control pressure.
In the past, attempts have been made to compensate for the hydrostatic head of control pressure fluid through the use of well fluid pressure to balance the hydrostatic head which tends to inhibit valve closure. The use of well fluid pressure to balance the valve, however, requires contact between well fluids and internal components of the safety valve. Since well fluids may contain abrasive and corrosive materials that tend to impede the function of the equipment, this approach presents considerable drawbacks in terms of equipment durability and reliability. Further, the use of well pressure to balance the valve against the hydrostatic head of the control line requires additional components in terms of specialized seals designed to protect the valve components. Additionally, due to the corrosive and abrasive nature of well fluids, this approach may require the use of special materials to manufacture valve components capable of withstanding contact with well fluids.
Thus, there exists a need for an improved surface controlled pressure balanced subsurface safety valve that overcomes the limitations of the prior art.